1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons contained in an exhaust gas discharged, for example, from internal combustion engines of automobiles. More specifically, it relates to a catalyst for removing nitrogen oxides contained in an oxygen-rich exhaust gas. Additionally, this invention relates to an adsorber/catalyst for removing nitrogen oxides contained in an oxygen-rich exhaust gas.
2. Background and Discussion of the Related Art
Air pollution due to nitrogen oxide (NO) and nitrous oxide (N2O) poses serious environmental problems. Whereas, nitrogen oxides have been implicated in acid rain and smog formation, NO2 is a greenhouse gas that contributes to the catalytic destruction of stratospheric ozone. As a result, abating these gases (NO and N2O) has become a matter of urgent environmental concern.
Anthropogenic activities appear to be largely responsible for the increased level of NO and N2O observed in the atmosphere. The reason being, both gases are emissions from automobile exhaust and several chemical processes. Nitrogen oxides, carbon monoxide and hydrocarbons, contained in an exhaust gas discharged from internal combustion engines, are removed, for example, through the use of a three-way catalyst comprising Pt, Rh, Pd, etc., supported on a carrier material. In the case of an exhaust gas discharged from diesel engines, however, no effective catalyst exists for removing nitrogen oxides due to the large amount of oxygen in the exhaust gas, a condition under which the typical TWC becomes inefficient. As such, purification of the exhaust gas by a TWC catalyst is not practical.
Recently lean-burn combustion gasoline engines have been utilized in an attempt to lower the fuel consumption and reduce the amount of exhausted carbon dioxide gas. However, exhaust gas from lean burn gasoline engines typically exhibit an atmosphere containing an excessive amount of oxygen, and thus again the utilization of the conventional three-way catalyst, is impractical.
One current technology that has been utilized to remove undesired nitrogen oxide pollutants from lean burning gasoline engines and diesel engines involves the use of adsorbers comprising cordierite honeycomb containing xcex3-alumina supported washcoat materials, in combination with the cycling of the engine between lean and rich bum conditions. Typically, these adsorbers consist of base metal oxides in combination with precious metals such as Pt. The mechanism of removal involves the adsorbed nitric oxide being oxidized to NO2 by the Pt catalyst during the lean operation of the vehicle. Subsequently, the nitrogen dioxide is chemically adsorbed on the base metal oxides to form solid nitrates. In order to regenerate the base metal oxide catalysts and to convert the by-products of the reaction to N2 rather than to undesirable nitrogen oxide species, the engine management system programs a rich cycle during which the nitrate salts release NO2 which is reacted over the Pt catalyst in the presence of excess reducing agents (excess hydrocarbons, CO, and H2) to form N2, H2O, and CO2. Although the use of NO adsorbers in a lean-rich cycle for diesel and lean bum gasoline engines has been demonstrated to increased NO conversions to N2, over a temperature range of about 250xc2x0 C. to 475xc2x0 C., this approach does have some drawbacks. Specifically, the following: (1) the rich cycle adversely affect the drivability for diesel engines and involves subjecting the engine to a fuel penalty; (2) the base metal oxide NO adsorbers are poisoned by SO2 and must be regenerated in a rich cycle.
Another approach to controlling nitrogen oxide emissions, has been the use of Cu-ZSM-5 zeolite catalysts. Although these catalysts have demonstrated adequate conversions of NO to N2, these catalysts typically suffer decreased conversion of NO as a result of aging in atmospheres having levels of steam in the 10-12% range found in combustion exhaust gas. Furthermore, even though zeolite catalysts are preferred over base metal oxide adsorbers, due to their direct conversion of NO to nitrogen and greater SO2 tolerance, these zeolite catalysts still result in a fuel penalty that is necessary to achieve in the aforementioned NO conversion.
One solution to the aforementioned excess oxygen problem, set forth in Japanese Unexamined Patent Publication (Kokai) Nos. 63-283727 and 1-130735, involves zeolite catalysts subjected to an ion exchange with a transition metal. These materials are capable of removing nitrogen oxides in an exhaust gas containing an excessive amount of oxygen without the addition of a special reducing agent such as ammonia. Specifically, these references propose a catalyst that can selectively reduce nitrogen oxides even in an exhaust gas containing an excessive amount of oxygen and can remove minor amounts of reducing agents such as unburnt carbon monoxide and hydrocarbons. While capable of removing nitrogen oxides, the ion-exchanged zeolites are subject to deterioration when used at a high temperature for a long time.
Accordingly, to solve the above-described problems, a catalyst for purifying an exhaust gas comprising a zeolite having an SiO2/Al2O3 mole ratio of at least 15, and incorporated therein, cobalt and a rare earth metal has been proposed (see Japanese Patent Application No. 2-149203). Although the exhaust gas purification catalyst proposed in Japanese Patent Application No. 2-149203 has an improved durability, the temperature region in which the nitrogen oxides can be removed is relatively narrow.
Lastly, U.S. Pat. No. 5,206,196 discloses an catalyst for removing nitrogen oxides from an oxygen-rich exhaust gas comprising a zeolite having a SiO2/Al2O3 ratio of at least 15 with a combination of a cobalt, a rare earth metal and silver incorporated in the zeolite. Although this catalyst capably removes nitrogen oxides from an oxygen-rich exhaust gas, the preparation of these materials is complex and expensive as a result of the incorporation of silver into the zeolite catalyst
There is, accordingly, a clear need for, and thus an object of the present invention is to provide, a catalyst capable sufficiently removing nitrogen oxides from an oxygen-rich exhaust gas without requiring complex and expensive formation procedures.
Accordingly, the object of the present invention is to solve the above problems of the prior art and to provide for a catalyst for removing nitrogen oxides from an oxygen-rich exhaust gas that is durable, exhibits high conversion activity and is relatively simple and inexpensive to produce.
Specifically, the invention is directed at a catalyst for purifying an exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons from exhaust gas comprising a zeolite having a silica/alumina molar ratio of greater than 55, and at least 1.5%, by weight, each of nickel and cobalt incorporated therein.
The invention is also directed at an adsorber/catalyst system for use in the removal of the oxides of nitrogen from waste gas, the adsorber/catalyst system comprising the following components: (1) a zeolite having a silica/alumina ratio of greater than 55; (2) at least 1.5%, by weight, each of nickel and cobalt incorporated therein; and, (3) an adsorber component comprising a mixture of a gamma alumina support and an adsorber material selected from the group consisting of metal oxides, metal hydroxides, metal carbonates mixed metal oxides and pillared clays.